In Thursday's fighting, more than 300 people were killed, the majority of whom were civilians, making it the heaviest toll yet in 16 months of fighting, the Britain-based Syrian Observatory for Human Rights said on Friday.

The release of the refugee figures came as the UN agency's High Commissioner expressed concern for Syrians forced to flee the country and those displaced within the country's borders.

"With the spread of deadly violence, I am gravely concerned for the thousands of Syrian refugees who have been forced to flee their homes," he said.

"I fear for the civilians caught up in the violence in Damascus, including the large Iraqi population residing there."

The UN agency said it was unable today to give an exact number of displaced people in Syria but estimated last week that "one million people may have been forced to flee inside the country since the conflict began".